Hi m.salsal,
as anshad indicated, I also advice you a portable w8,
customization capabilities comes from Chris and JFX,
and all people who shared their feedbacks around, not winbuilder ;) ,
people who have specific tasks, like you, when they figure out, they share with others,
there are many plugins Chris made available that increase flexibility, including from feedbacks and his findings,
winbuilder is only a tool full of bugs we are using, creating headache, but we don't have time or sources to replace with.
For your specific goal (with portable w8 or SE),
I feel what you need is, for your
specific task, only removing diskstorage driver(s) from your build

OR maybe, I feel removing disk.sys would remove all disk access including ufd, if so there should be a way to remove fixed drivers ;) ) -> test and see ;)
Another idea, I am not sure but, maybe removing untfs.dll may remove ntfs support, hence only fat and exfat supported (via usb flash disk) -> test and see ;)
only ideas to test, probably there will be more when you look around internet, let us know results if you continue that way,
besides don't insist to ask,
there is no feature available today, if you or anyone else figure out and give feedback, there will be

